Attempts at delivering curative doses of ionizing radiation to malignant tumors in the abdomen, have been associated with serious complications from the gastrointestinal tract. The resultant patient morbidity and mortality rates have been high. The limiting factor is the low tolerance of the normal gastrointestinal tract to ionizing radiation. To overcome this problem, attempts have been made to render the bowel ischemic, in order to increase the "therapeutic ratio" between tumors and the normal tissues. Vasopressin decreases mesenteric blood flow and tissue oxygen consumption. When infused intraarterially, it provides a degree of bowel protection from ionizing radiation. The method, however, is not clinically applicable because radiation is delivered in fractions over 6-8 weeks. The principal investigator has preliminary data to suggest that intravenous vasopressin at low infusion rates can provide substantial bowel protection from ionizing radiation. This would be further pursued with the proposed study, which has the following specific objectives: (1) To confirm the radioprotective properties of intravenous vasopressin during acute and fractionated irradiation. (2) To establish the optimal dose rate and duration of intravenous infusions of vasopressin. (3) To study potential undesirable side effects of such infusions. These specific objectives will be pursued with experiments in dogs. Mesenteric blood flow, cardiac output and oxygen consumption will be studied, with varying dose rates of intravenous vasopressin. The "optimal" dose-rate will then be applied to dogs undergoing acute or fractionated abdominal irradiation.